1. Field of the Invention
The present invention relates to perpendicular magnetic recording media using a ferromagnetic thin film having perpendicular magnetic anisotropy for a recording layer and a magnetic recording apparatus using it.
2. Description of the Related Art
Recently, information contents have been strikingly increased and as a result, the storage capacity of a magnetic recording apparatus used for a mass storage file of a computer has been rapidly enhanced.
In magnetic recording, the recording of information is realized by inverting the orientation of magnetization in a minute region of a ferromagnetic thin film to be a recording medium. Currently, a so-called longitudinal recording type in which the orientation of magnetization is parallel to the surface of a medium is widely used. In this recording type, the density is enhanced by enhancing the coercivity of a medium or by reducing the thickness of a magnetic layer. However, the current thickness of a longitudinal recording medium is already 30 nm or less, and advanced sputtering technique by which a film having tribological strength and having few defects is formed is required for further thinning a film. In addition, to prevent signal to noise ratio from being deteriorated by enhancing the density, crystal grains forming a recording thin film are required to be made smaller. However, making grain size smaller means that the resistance against thermal disturbance is weakened and so-called thermal decay by which information recorded on a medium decays as time goes is promoted.
For a method of solving the above, a perpendicular recording in which magnetization is in a perpendicular direction to the surface of a medium in recording is proposed. In this type, material having strong magnetic anisotropy with its easy axis perpendicular direction to the surface of a film is required, and a relatively thick alloy magnetic film, typically a Coxe2x80x94Cr alloy thin film having the thickness of 0.01 to 0.5 xcexcm is widely used. This recording type is excellent in that the higher recording density is, the stabler magnetization for recording becomes from the viewpoint of energy, and the recording type is essentially suitable for high density recording. In addition, this recording type has an advantage that the thickness of a film is not required to be reduced and the coercivity is not required to be enhanced to enhance recording density as is the case in longitudinal recording. In addition, it means that the volume of a micro crystal grain forming the thin film can be set to a larger value, compared with that in longitudinal recording, because the thickness of the film is not required to be reduced, which means the this recording type also has excellent resistance against thermal fluctuation.
For media for this perpendicular magnetic recording, as disclosed in Japanese Laid-Open No. 57-109127, on pp. 57 to 60 of Japan Applied Magnetic Congress Issue vol. 9 No. 2 (1985), on IEEE Trans., MAG-24, No. 6, pp. 2706 to 2708 (1988) and others, a Coxe2x80x94Cr alloy thin film is used and it is desirable that a non-magnetic Cr atom is segregated at the boundaries between micro crystal grains forming the medium. The reason is considered to be that corrosion resistance is enhanced by forming a region in which the density of Cr is high in a grain boundary, and that by segregating a non-magnetic Cr atom at the grain boundaries magnetic exchange interaction between grains being cut off, as in the case of longitudinal recording media, the magnetic domains are made smaller and media noise is reduced.
In the meantime, for media for perpendicular recording other than the Coxe2x80x94Cr alloy thin film, there is an amorphous TbFe thin film used for media for magneto-optical recording. This type of film has large magnetic anisotropy, but, exchange interaction between grains is strong quite differently from the Coxe2x80x94Cr film. Therefore, a magnetization curve is an ideal square and its squareness value is substantially close to one.
Generally, when magnetic recording is performed on such a film using a magnetic head, d.c. demagnetization noise is low and excellent signal to noise ratio is acquired in a low density region.
In Japanese Laid-Open No. 9-91660, a magnetic recording medium wherein a first perpendicular magnetic recording film and a second perpendicular magnetic recording film respectively different in the characteristic are laminated is disclosed.
Also, in Japanese Laid-Open No. 10-334440, a magnetic recording medium is disclosed wherein a second perpendicular magnetic recording film in which the magnetic coupling in an in-plane direction is relatively strong is formed on the surface of a Co alloy perpendicular magnetic recording film which is a main recording layer and micro magnetization fluctuation existing on the surface of the main recording layer is reduced.
Though noise is small in prior art in which the Coxe2x80x94Cr alloy thin film is used and a Cr atom is segregated at the grain boundaries, sufficient consideration is not given to fabrication of a medium resistant to thermal fluctuation.
Also, in prior art using an amorphous TbFe thin film used for media for magneto-optical recording, excellent signal to noise ratio is acquired in a low density region, but, sufficient consideration is not given to the fact that recording properties rapidly deteriorate when recording density becomes high.
Also, in prior art described in the Japanese published unexamined patent applications No. Hei 9-91660 and No. Hei 10-334440, the first magnetic recording film has polycrystalline structure, but, the second magnetic recording film has multilayer structure or amorphous structure and the structure of the two films is considerably different and sufficient consideration is not given to industrial manufacturing facility.
The object of the invention is to provide perpendicular magnetic recording media which has excellent signal to noise ratio in a wide recording density region, also has stable properties against thermal fluctuation and can be easily manufactured, and a magnetic recording apparatus using them.
One means to achieve the object is to use perpendicular magnetic recording media according to the invention which include a non-magnetic substrate, a first recording layer which is provided on the non-magnetic substrate, which has perpendicular magnetic anisotropy and in which he magnitude of magnetic exchange interaction between grains is substantially zero, and a second recording layer which is laminated on the first recording layer, which has perpendicular magnetic anisotropy and in which magnetic exchange interaction between grains is larger than that in the first recording layer.
It is desirable that an exchange stiffness constant between grains of the first recording layer is 0.05xc3x9710xe2x88x9211 J/m or less and an exchange stiffness constant between grains of the second recording layer is in a range of 0.15xc3x9710xe2x88x9211 J/m to 0.8xc3x9710xe2x88x9211 J/m. The lower limit of the exchange stiffness constant between grains of the first recording layer may be also zero, however, as it is difficult to find a magnetic film the exchange stiffness constant of which is zero, it is desirable that the exchange stiffness constant is 0.0001xc3x9710xe2x88x9211 J/m or more. Also, it is desirable that the magnetic anisotropy constant of the second recording layer is in a range of 2.5xc3x97105 J/m3 to 4.5xc3x97105 J/m3. Further, it is desirable that the second recording layer has polycrystalline structure.
Another means to achieve the object according to use perpendicular magnetic recording media according to this invention which include a non-magnetic substrate, a first recording layer provided on the non-magnetic substrate and provided with perpendicular magnetic anisotropy and a second recording layer laminated on the first recording layer and provided with perpendicular magnetic anisotropy. The exchange stiffness constant, which shows the magnitude of magnetic exchange interaction between grains of the first recording layer, is 0.05xc3x9710xe2x88x9211 J/m or less and 0.0001xc3x9710xe2x88x9211 J/m or more.
It is desirable that an exchange stiffness constant between grains of the second recording layer is in a range of 0.15xc3x9710xe2x88x9211 J/m to 0.8xc3x9710xe2x88x9211 J/m.
Still other means to achieve the object is to use perpendicular magnetic recording media according to this invention which include a non-magnetic substrate, a first recording layer which is provided on the non-magnetic substrate, which is a magnetic film mainly made of Co and Cr, which has perpendicular magnetic anisotropy and the magnitude of magnetic exchange interaction between grains of which is substantially zero, and a second recording layer laminated on the first recording layer and provided with perpendicular magnetic anisotropy. The Cr is segregated at the grain boundaries between ferromagnetic grains in the first recording layer, and the Cr density is in a range of 21 atomic percents to 29 atomic percents.
It is desirable that an exchange stiffness constant between grains in the first recording layer is 0.05xc3x9710xe2x88x9211 J/m or less. The lower limit of the value may be also zero, however, as it is difficult to find a magnetic film the exchange stiffness constant of which is zero, it is desirable that the exchange stiffness constant is 0.0001xc3x9710xe2x88x9211 J/m or more.
Also, it is desirable that the second recording layer is a magnetic film mainly made of Co and Cr and an exchange stiffness constant between grains is in a range of 0.15xc3x9710xe2x88x9211 J/m to 0.8xc3x9710xe2x88x9211 J/m. It is desirable that Cr is segregated at the grain boundaries between ferromagnetic grains in the second recording layer, and the Cr density is in a range of 16 atomic percents to 19 atomic percents.
In all perpendicular magnetic recording media described above, it is desirable that the sum of the thickness of the first recording layer and the thickness of the second recording layer is 10 to 100 nm and the ratio of the thickness of the second recording layer to the thickness of the first recording layer is in a range of 0.5 to 1.0. The first recording layer is mainly made of Co and Cr, and Ta, B or Pt may be added. The second recording layer may be made of Co, Cr and Pt.